Why Internal Resistance Confuses Everyone (And How to Actually Understand It)

“Your battery says 9V… so why does your circuit only get 7.8V? Where did the rest go?”

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The Hidden Concept That Costs Marks

Internal resistance is one of those A-Level Physics topics that looks simple—until students hit exam questions.

They can often:

• Rearrange equations ✔
• Do calculations ✔
• Recognise circuits ✔

But ask them what’s actually happening, and things quickly fall apart.

That’s because internal resistance isn’t just maths.

It’s energy, physics, and real-world behaviour all wrapped into one.

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EMF vs Terminal Potential Difference (The Core Confusion)

This is where most problems begin.

EMF (ε)

• The total energy supplied per unit charge
• What the battery could provide
• Measured when no current flows

Terminal Potential Difference (V)

• The actual energy delivered to the circuit
• What the components really get
• Measured when current is flowing

The Key Idea

The battery does not give all its energy to the circuit.

Some is lost inside the battery itself.

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Where Does the “Lost Voltage” Go?

That missing voltage isn’t “lost” in a mysterious way.

It’s converted into heat inside the battery.

Inside every cell is resistance—just like a resistor in your circuit.

So when current flows:

• Energy is transferred inside the battery
• The battery warms up (sometimes noticeably)
• Less energy reaches the external circuit

The Equation Behind It

$$𝑉=𝜀-𝐼𝑟$$

Where:

• $𝜀$= EMF
• $𝐼$= current
• $𝑟$ = internal resistance

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Why Voltage Drops Under Load

When no current flows:

• $𝐼=0$
  
• $𝑉=𝜀$
  

But as soon as you connect a circuit:

• Current flows
• The term $𝐼𝑟$ increases
• Terminal voltage drops

Simple way to think about it:

The harder the battery works (more current), the more energy it wastes internally.

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The Practical (Where It Finally Clicks)

This is where your teaching setup really shines.

Students understand internal resistance when they see it happening.

Practical approach:

• Use a variable resistor to change current
• Measure:
  • Current (I)
  • Terminal voltage (V)
• Plot a graph of V vs I

What they observe:

• A straight line
• Negative gradient = internal resistance (r)
• Y-intercept = EMF (ε)

Suddenly, it’s not abstract anymore—it’s measurable.

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Common Exam Mistakes (And How to Fix Them)

1. Mixing up EMF and voltage

Students treat them as the same thing.

✔ Fix:

• Always ask: Is current flowing?
• If yes → it’s terminal p.d., not EMF

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2. Ignoring internal resistance entirely

Students use $𝑉=𝐼𝑅$ blindly.

✔ Fix:

• Look for clues:
  • “Battery”
  • “Cell”
  • “Terminal voltage”
• These usually signal internal resistance is involved

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3. Not interpreting graphs properly

Students can plot but not explain.

✔ Fix:

• Practise linking:
  • Gradient → internal resistance
  • Intercept → EMF

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4. No physical understanding

They calculate correctly—but don’t explain energy loss.

✔ Fix:

• Use phrases like:

  “Energy is dissipated as heat within the cell due to internal resistance.”

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The Big Picture

Internal resistance isn’t just an exam topic.

It explains:

• Why batteries get warm
• Why devices lose efficiency
• Why high currents are problematic
• Why real circuits never behave perfectly

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Final Thought

Once students stop seeing internal resistance as just an equation and start seeing it as:

energy being shared between the circuit and the battery itself

Everything clicks.